1. Field of the Invention
The present invention relates to a continuous blood-pressure monitoring apparatus which includes a pressure-pulse-wave sensor adapted to be pressed against an artery of a living subject via the skin and continuously monitors blood pressure of the subject based on a pressure pulse wave detected by the sensor.
2. Related Art Statement
There is known a continuous blood-pressure monitoring apparatus which includes an inflatable cuff adapted to be worn on a portion of a living subject; a blood-pressure determining means for determining a blood pressure of the subject based on a signal obtained while a pressure in the cuff is slowly changed; a relationship determining means for determining, in advance, a relationship between blood pressure and magnitude of pressure pulse wave, based on the blood pressure determined by the blood-pressure determining means and a magnitude of a pressure pulse wave detected from an artery of the subject; and a blood-pressure monitoring means for successively determining, according to the thus determined relationship, a blood-pressure value of the subject based on each of respective magnitudes of the pressure pulse wave detected from the artery. Since the blood-pressure values successively determined by the continuous blood-pressure monitoring apparatus are very highly reliable, the apparatus can be used in those cases in which strict blood-pressure monitoring is needed. This apparatus is disclosed in, e.g., Japanese Utility Model Document No. 2-82309 or its corresponding U.S. Pat. No. 5,139,026.
In the conventional continuous blood-pressure monitoring apparatus, disclosed in the above-indicated document, a pressure-pulse-wave detecting device for detecting the pressure pulse wave employs a pressure-pulse-wave sensor which is worn on a wrist of a living subject and is pressed against a radial artery of the wrist. In this case, a condition under which the pressure-pulse-wave sensor is pressed against the artery may be changed by, e.g., a change of a state in which the sensor is worn, caused by a physical motion of the subject. Hence, in order to increase the reliability of blood-pressure values determined by the blood-pressure monitoring means, a calibration is periodically carried out to update the relationship between blood pressure and magnitude of pressure pulse wave. In each calibration, the blood-pressure determining means determines a new blood pressure of the subject in a process in which the pressure of the cuff is changed in a prescribed manner, and the relationship determining means determines a new relationship between blood pressure and magnitude of pressure pulse wave, based on the new blood pressure determined by the blood-pressure determining means and a magnitude of the pressure pulse wave detected by the pressure-pulse-wave sensor during the above-indicated process.
However, when each calibration is carried out, the cuff is inflated to press the subject""s body, thereby causing the subject to feel discomfort. In addition, the calibration is periodically carried out irrespective of whether the condition under which the sensor is pressed is appropriate or not, and the calibration period needs to be shortened to increase the reliability of continuous monitoring of blood pressure. Thus, the burden exerted on the subject is increased.
In order to solve the above-indicated problem, Japanese Patent Document No. 7-284479 or its corresponding U.S. Pat. No. 5,860,932 discloses a continuous blood-pressure monitoring apparatus in which a pressure-pulse-wave sensor is worn on a portion of a subject that is located on a downstream side of a cuff, a pressure of the cuff is increased at a prescribed rate, and a judging means judges whether a relationship between blood pressure and pressure-pulse-wave magnitude is appropriate, based on a shape or an area of a pressure pulse wave detected by the sensor during the increasing of the cuff pressure. More specifically described, the disclosed apparatus determines, in a state in which the cuff pressure would be substantially equal to, or somewhat higher than, a diastolic blood pressure of the subject, the diastolic blood pressure of the subject by utilizing the fact that the tendency of change of respective shapes or areas of respective heartbeat-synchronous pulses of the pressure pulse wave, successively detected by the sensor during the increasing of the cuff pressure, significantly changes around the diastolic blood pressure, compares the thus determined diastolic blood pressure with a diastolic blood pressure determined based on a magnitude of a heartbeat-synchronous pulse of the pressure pulse wave according to the relationship between blood pressure and pressure-pulse-wave magnitude, and judges whether the relationship is appropriate. If it is judged that the relationship is appropriate, then it is not needed to carry out a calibration, which leads to reducing the discomfort the subject suffers.
However, even in the above-described continuous blood-pressure monitoring apparatus, the cuff pressure is increased up to a value substantially equal to, or somewhat higher than, the diastolic blood pressure, so as to judge whether the relationship is appropriate or not. Thus, the discomfort the subject suffers is not sufficiently reduced.
In addition, in the above-described continuous blood-pressure monitoring apparatus, the pressure-pulse-wave sensor needs to be worn on the subject""s portion located on the downstream side of the cuff. On the other hand, in many cases, the continuous blood-pressure monitoring apparatus is used during a surgical operation or in an intensive care unit when or where many devices are connected to the subject and, in some cases, the pressure-pulse-wave sensor cannot be worn on the subject""s portion located on the downstream side of the cuff.
It is therefore an object of the present invention to provide a continuous blood-pressure monitoring apparatus which sufficiently largely reduces discomfort felt by a living subject and sufficiently largely increases freedom of wearing.
The Inventor has carried out extensive studies to achieve the above-indicated object, and has found that pulse-wave-propagation-velocity-related information, such as a pulse-wave propagation time or a pulse-wave propagation velocity, that is related to a velocity at which a pulse wave propagates in a living subject, is not so accurate as a monitor blood pressure determined by a blood-pressure monitoring means based on a pressure pulse wave detected by a pressure-pulse-wave detecting probe from the subject, but it does change in relation with a change of the blood pressure of the subject, and that if the comparison between a change of the monitor blood pressure and a change of the pulse-wave-propagation-velocity-related information obtained concurrently with the monitor blood pressure shows that the difference between the two changes is not so great, it can be judged that a pressure-pulse-wave sensor of the pressure-pulse-wave detecting device is appropriately pressed. In addition, if a pressure pulse wave is detected from a portion of the subject that is distant from the chest of the subject, the detected pressure pulse wave contains a heart-sound component. That is, since the single sensor can detect the two heartbeat-synchronous signals, i.e., the heart sounds and the pulse wave, the pulse-wave-propagation-velocity-related information can be obtained by using the single sensor worn on the subject.
Moreover, the Inventor has found that if the heart-sound component is extracted from the pressure pulse wave detected to determine the monitor blood pressure, and the pulse-wave-propagation-velocity-related information is obtained based on the pressure pulse wave detected to determine the monitor blood pressure, and the heart-sound component extracted from the pressure pulse wave, it is not needed to wear, on the subject, another sensor to obtain the pulse-wave-propagation-velocity-related information. That is, the sensor to obtain the pulse-wave-propagation-velocity-related information may be the same as, or different from, the pressure-pulse-wave sensor to detect the pressure pulse wave used to determine the monitor blood pressure. The present invention has been developed based on these findings.
The above object has been achieved by the present invention. According to a first aspect of the present invention, there is provided an apparatus for continuously monitoring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a portion of the subject; a blood-pressure determining means for determining a blood pressure of the subject based on a signal obtained while a pressure in the cuff is changed; a relationship determining means for determining a relationship between blood pressure and magnitude of pressure pulse wave, based on the blood pressure determined by the blood-pressure determining means and a magnitude of a first-artery pressure pulse wave detected from a first artery of the subject; a blood-pressure monitoring means for iteratively determining, according to the thus determined relationship, a monitor blood-pressure value of the subject based on a magnitude of each of a plurality of heartbeat-synchronous pulses of the first-artery pressure pulse wave detected from the first artery; a first pressure-pulse-wave sensor which is adapted to be worn on a portion of the subject that is distant from a chest of the subject, detects a second-artery pressure pulse wave that is produced by a second artery of the portion, and produces a pressure-pulse-wave signal representing the detected second-artery pressure pulse wave; a heart-sound extracting means for extracting, from the pressure-pulse-wave signal produced by the pressure-pulse-wave sensor, a heart-sound component representing heart sounds generated by a heart of the subject; a pulse-wave-propagation-velocity-related-information obtaining means for iteratively obtaining a piece of pulse-wave-propagation-velocity-related information which is related to a velocity at which the second-artery pressure pulse wave propagates through the second artery of the subject, based on a time of occurrence of a prescribed periodic point of a heartbeat-synchronous pulse of the heart-sound component extracted by the heart-sound extracting means and a time of occurrence of a prescribed periodic point of a corresponding heartbeat-synchronous pulse of the second-artery pressure pulse wave detected by the pressure-pulse-wave sensor; a propagation-velocity-related-information-change-value determining means for periodically determining, at a prescribed judgment period, a change value of the pieces of pulse-wave-propagation-velocity-related information obtained by the pulse-wave-propagation-velocity-related-information obtaining means; a monitor-blood-pressure-change-value determining means for periodically determining, at the judgment period, a change value of the monitor blood-pressure values determined by the blood-pressure monitoring means; and a relationship checking means for comparing the change value of the pieces of pulse-wave-propagation-velocity-related information, determined by the propagation-velocity-related-information-change-value determining means, and the change value of the monitor blood-pressure values, determined by the monitor-blood-pressure-change-value determining means, with each other, and thereby judging whether the relationship between blood pressure and magnitude of pressure pulse wave, determined by the relationship determining means, is appropriate. The first and second arteries may be the same as, or different from, each other, and the first-artery and second-artery pressure pulse waves may be the same as, or different from, each other.
According to this feature, the heart-sound extracting means extracts the heart-sound component from the pressure pulse wave detected by the first pressure-pulse-wave sensor; the pulse-wave-propagation-velocity-related-information obtaining means obtains the pulse-wave-propagation-velocity-related information based on the heart-sound component and the pressure pulse wave detected by the first pressure-pulse-wave sensor; and the propagation-velocity-related-information-change-value determining means determines, at the prescribed judgment period, the change value of the pulse-wave-propagation-velocity-related information. Since the pulse-wave-propagation-velocity-related information changes with the change of blood pressure of the subject, the change value of the pulse-wave-propagation-velocity-related information also changes with the change of blood pressure. In addition, the change value of the monitor blood-pressure value determined by the monitor-blood-pressure-change-value determining means also changes with the change of blood pressure. In the case where the condition under which a pressure-pulse-wave detecting device is worn on the subject has changed and the monitor blood-pressure value determined by the blood-pressure monitoring means is not accurate, the change value of the monitor blood-pressure value largely differs from the change value of the pulse-wave-propagation-velocity-related information. Therefore, the relationship checking means compares the change value of the monitor blood-pressure value and the change value of the pulse-wave-propagation-velocity-related information, with each other, and judges whether the relationship between blood pressure and pressure pulse wave, determined by the relationship determining means, is appropriate. Therefore, a longer period can be employed at which the blood-pressure determining means is operated to update the relationship between blood pressure and pressure pulse wave, and accordingly the discomfort the subject feels can be reduced. Moreover, since the pulse-wave-propagation-velocity-related information can be obtained by using only the first pressure-pulse-wave sensor worn on the subject, and it is not needed to wear the first pressure-pulse-wave sensor on the downstream side of the cuff, the first pressure-pulse-wave sensor enjoys an increased freedom of wearing.
According to a second aspect of the present invention, there is provided an apparatus for continuously monitoring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a portion of the subject; a blood-pressure determining means for determining a blood pressure of the subject based on a signal obtained while a pressure in the cuff is changed; a first relationship determining means for determining a relationship between blood pressure and magnitude of pressure pulse wave, based on the blood pressure determined by the blood-pressure determining means and a magnitude of a first-artery pressure pulse wave detected from a first artery of the subject; a blood-pressure monitoring means for iteratively determining, according to the thus determined relationship, a monitor blood-pressure value of the subject based on a magnitude of each of a plurality of heartbeat-synchronous pulses of the first-artery pressure pulse wave detected from the first artery; a first pressure-pulse-wave sensor which is adapted to be worn on a portion of the subject that is distant from a chest of the subject, detects a second-artery pressure pulse wave that is produced by a second artery of the portion, and produces a pressure-pulse-wave signal representing the detected second-artery pressure pulse wave; a heart-sound extracting means for extracting, from the pressure-pulse-wave signal produced by the pressure-pulse-wave sensor, a heart-sound component representing heart sounds generated by a heart of the subject; a standard-pulse-wave-propagation-velocity-related-information obtaining means for obtaining, as a standard piece of pulse-wave-propagation-velocity-related information, a piece of pulse-wave-propagation-velocity-related information which is related to a velocity at which the second-artery pressure pulse wave propagates through the second artery of the subject, based on a time of occurrence of a prescribed periodic point of a heartbeat-synchronous pulse of the heart-sound component in a first time duration comprising at least one of a first time period in which the pressure of the cuff is changed, a prescribed preceding time period preceding the first time period, and a prescribed following time period following the first time period, and a time of occurrence of a prescribed periodic point of a corresponding heartbeat-synchronous pulse of the second-artery pressure pulse wave in the first time duration; a second relationship determining means for determining a second relationship between blood pressure and pulse-wave-propagation-velocity-related information, based on the blood pressure determined by the blood-pressure determining means and the standard piece of pulse-wave-propagation-velocity-related information obtained by the standard-pulse-wave-propagation-velocity-related-information obtaining means; a judgment-pulse-wave-propagation-velocity-related-information obtaining means for obtaining, as a judgment piece of pulse-wave-propagation-velocity-related information, a piece of pulse-wave-propagation-velocity-related information which is related to the velocity at which the second-artery pulse wave propagates through the second artery of the subject, based on a time of occurrence of a prescribed periodic point of a heartbeat-synchronous pulse of the heart-sound component extracted by the heart-sound extracting means, and a time of occurrence of a prescribed periodic point of a corresponding heartbeat-synchronous pulse of the second-artery pressure pulse wave detected by the pressure-pulse-wave sensor, at a prescribed judgment period; an estimated-blood-pressure determining means for determining, according to the second relationship, an estimated blood-pressure value of the subject based on the judgment piece of pulse-wave-propagation-velocity-related information obtained by the judgment-pulse-wave-propagation-velocity-related-information obtaining means; and a relationship checking means for comparing, at the judgment period, the estimated blood-pressure value determined by the estimated-blood-pressure determining means, and a monitor blood-pressure value determined by the blood-pressure monitoring means based on a magnitude of a heartbeat-synchronous pulse of the first-artery pressure pulse wave detected at a time around a time when the judgment piece of pulse-wave-propagation-velocity-related information is obtained by the judgment-pulse-wave-propagation-velocity-related-information obtaining means, with each other, and thereby judging whether the relationship between blood pressure and magnitude of pressure pulse wave, determined by the first relationship determining means, is appropriate.
According to this feature, the estimated-blood-pressure determining means determines the estimated blood-pressure value based on the second pulse-wave-propagation velocity-related information, according to the relationship between blood pressure and pulse-wave-propagation-velocity-related information. If the condition under which a pressure-pulse-wave detecting device is worn on the subject has changed and accordingly the monitor blood-pressure value determined by the blood-pressure monitoring means is not accurate, the monitor blood-pressure value largely differs from the estimated blood-pressure value. Hence, the relationship checking means compares the estimated blood-pressure value and the monitor blood-pressure value determined by the blood-pressure monitoring means, with each other, and judges whether the relationship between blood pressure and pressure pulse wave, determined by the first relationship determining means, is appropriate or not. Therefore, a longer period can be employed at which the blood-pressure determining means is operated to update the relationship between blood pressure and pressure pulse wave, and accordingly the discomfort the subject feels can be reduced. Moreover, since the standard pulse-wave-propagation-velocity-related information or the judgment pulse-wave-propagation-velocity-related information can be obtained by using only the first pressure-pulse-wave sensor worn on the subject, and it is not needed to wear the first pressure-pulse-wave sensor on the downstream side of the cuff, the first pressure-pulse-wave sensor enjoys an increased freedom of wearing.
According to a preferred feature of the present invention, the apparatus further comprises a second pressure-pulse-wave sensor which is adapted to be pressed against the first artery of the subject and detects the first-artery pressure pulse wave generated by the first artery, and the first pressure-pulse-wave sensor is adapted to be worn on the portion of the subject that is more proximal to the heart of the subject than the first artery against which the second pressure-pulse-wave sensor is adapted to be pressed.